Nutrient and carbon balances in organic vegetable production on an irrigated, sandy soil in northern Oman

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AbstractOur understanding of nutrient and carbon (C) fluxes in irrigated organic cropping systems of subtropical regions is limited. Therefore, leaching of mineral nitrogen (N) and phosphorus (P), gaseous emissions of NH3, N2O, CO2, and CH4, and total matter balances were measured over 24 months comprising a total cropping period of 260 d in an organic‐cropping‐systems experiment near Sohar (Oman). The experiment on an irrigated sandy soil with four replications comprised two manure types (ORG1 and ORG2) characterized by respective C : N ratios of 19 and 25 and neutral detergent fiber (NDF)‐to‐soluble carbohydrates (SC) ratios of 17 and 108. A mineral‐fertilizer (MIN) treatment with equivalent levels of mineral N, P, and potassium (K) served as a control. The three treatments were factorially combined with a cropping sequence comprising radish (Raphanus sativus L.) followed by cauliflower (Brassica oleracea L. var. botrytis) or carrot (Daucus carota subsp. sativus). Over the 24‐months experimental period gaseous N emissions averaged 45 kg ha–1 (59% NH3‐N, 41%N2O‐N) for MIN, 55 kg N ha–1 (69% NH3‐N, 31%N2O‐N) for ORG1, and 49 kg N ha–1 (59% NH3‐N, 41% N2O‐N) for ORG2. Carbon losses were 6.2 t ha–1 (98% CO2‐C, 2% CH4‐C) for MIN, 9.7 t C ha–1 (99% CO2‐C, 1% CH4‐C) for ORG1, and 10.6 t ha–1 (98% CO2‐C, 2% CH4‐C) for ORG2. Exchange resin–based cumulative leaching of mineral N amounted to 30 kg ha–1 for MIN, 10 kg ha–1 for ORG1, and 56 kg ha–1 for ORG2. Apparent surpluses of 361 kg N ha–1 and 196 kg P ha–1 for radish‐carrot and 299 kg N ha–1 and 184 kg P ha–1 for radish‐cauliflower were accompanied by K deficits of –59 kg ha–1 and –73 kg ha–1, respectively, for both cropping systems. Net C balances for MIN, ORG1, and ORG2 plots were –7.3, –3.1, and 1.5 t C ha–1 for radish‐carrot and –5.0, 1.3, and 4.6 t C ha–1 for radish‐cauliflower. The results underline the difficulty to maintain soil C levels in intensively cultivated, irrigated subtropical soils.